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TaN multilayer systems
~ Journalof -,~_" magnetism and magnetic ~ i materials ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 75-76
M6ssbauer investigation of FeTaN/TaN multilayer systems M.S. Rogalski a'b'*, M.M. Amado a, J.B. Sousa a, P.P.
Freitas c
alFIMUP and Faculdade de Ciencias, Universidade do Porto, Rua Campo Alegre 687, 4150 Porto, Portugal bUCEH, Universidade do Algarve, Gambelas, 8000 Faro, Portugal ClNESC and Instituto Superior Tbcnico, Rua Alves Redol 9, 1000 Lisbon, Portugal
Abstract
The microstructure of the [FeTaN(3200,~)/TaN(50 A)], multilayer system, prepared by DC reactive magnetron sputtering, is studied by transmission and conversion electron M6ssbauer spectroscopy, that indicate a predominant a-Fe texture and the presence of iron nitrides. Comparative phase analysis reveals formation of the paramagnetic nitride a-FexN(2 < x < 3) in the interfacial region between the FeTaN and TaN spacer layers. This phase, whose relative fraction increases in proportion to the square of Nz partial pressure during deposition, may account for the relatively large magnetostriction of the multilayer system. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Ferromagnetic multilayers; Conversion electron-MiSssbauer; Thin films-M6ssbauer spectroscopy; Magnet-
ronsputtering
FeTaN films exhibit a combination of high saturation magnetization and good soft magnetic-behaviour, which is suitable for pole piece materials of high density recording heads. Improved magnetic properties over single layer films have been found for multilayers, where FeTaN films are separated by high resistivity spacer layers [1]. Recently, a new multilayer system [FeTaN/TaN],, deposited by DC reactive magnetron sputtering, was reported as a good candidate for high magnetization pole piece fabrication, provided magnetostriction is lowered [2]. In the present paper, microstructure of [FeTaN/ TAN], multilayers was studied by transmission and conversion electron (CEMS) M6ssbauer spectroscopy and was found to be rather sensitive to the nitrogen partial pressure during deposition of soft FeTaN layers. The [FeTaN(3200 ~)/TaN(50 ~,)], multilayers were deposited, up to a total thickness of 1.6 ~tm, on water cooled mica substrates, using a Nordiko 2000 sputtering system. For FeTaN film deposition, 3 in Fe-10 wt% Ta alloy targets were used, in 3 mTorr Ar + Nz atmosphere, * Corresponding address. UCEH, Universidade do Algarve, Gambelas, 8000 Faro, Portugal, Fax: + 351-89-818560; e-mail: [email protected].
at variable nitrogen partial pressure PN2 from 0 to 0.12mTorr. An aligning field of 20 Oe was applied. High-resistivity TaN films were deposited from 3 in Ta targets in 5 mTorr Ar + N2 atmosphere, at constant PN2 = 0.19 mTorr. Transmission STFe M6ssbauer spectra were obtained at room temperature on a Wissel constant acceleration spectrometer which utilized a 57Co/Rh source. The CEMS measurements were performed at room temperature by using an He-4% CH 4 gas-flow electron counter (Rikon-5 Wissel). The resulting spectra were calibrated with a-Fe foil at room temperature and fitted using an integrated least-squares computer program. The information provided by the transmission M6ssbauer is integrated over the five FeTaN layers, including the interfacial regions between the magnetic and spacer layers, while that derived from the CEMS spectra is limited to a maximum depth of 3000 ~,, and hence is relevant for the upper FeTaN layer structure only. All transmission M6ssbauer spectra, given in Fig. la-e, show a dominant contribution from the magnetic sublattice with H F of 334 kOe, typical for a-Fe, with spins oriented in the plane of the FeTaN layers, as indicated by the 3 : 4 : 1 : 1 : 4 : 3 relative intensity ratio.
0304-8853/99/$- see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 85 3(9 8 ) 0 0 6 6 7 - 2
76
M S. Rogalski et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 75- 76
•
100
°
(a)
(b)
1.02 .Co~ 8 0 ! "
1.00 1.00 >,0.99
oo
1.00 '.;±-
~
60- • O 40 L V
• •
• •
,~Fe FeN (x>3) ,-Fe N (2
•
• •
• O
,~Fe FeN (x>3)
O
O O
• •I
I
-;':-. -=~
~> 0.99
20~O
(d)
oi_
Oo .
0 "~O 0'04
lO0
~
O O
-
0 "08 I O: ~ ~ 0 I00 0"64 0'68
0'12
PN [mTorr]
0.99 ,
1.00 ~ .:-...
b
"
.
"
0,99
,
L
.
.
~
I
,
,
~
Fig. 2. Fe phase concentration vs. nitrogen partial pressure PN, during the FeTaN layer deposition, calculated fi-om (a) transmission M6ssbauer spectra and (b) conversion electron M6ssbauer spectra.
1.00
0.99 -8
-6
-4
-2
0
2
4
6
Velocity [rnm/s]
Fig. 1. Transmission M6ssbauer spectra recorded from FeTaN/TaN multilayer system deposited at variable nitrogen partial pressure: (a) Pu~ = 0; (b) PN~ =0.05mTorr; (c) p.,~, = 0.07 mTorr; (d) PN~ = 0.10 mTorr; (e) p~ = 0.12 mTorr and the corresponding CEMS spectrum from the system deposited at p,~.: = 0.12 mTorr (f). Larger linewidths than those expected for metallic Fe are consistent with a spreading in magnitude of the H F values, which is consistent with the fine grain size calculated from the X-ray diffraction patterns [3]. A second magnetic phase, with H F = 295 kOe can be identified as iron nitride FexN, which is ferromagnetic at room temperature for x > 3 [4] and contains Fe magnetic moments also oriented in the plane of the film. Since the Curie temperature of Fe nitrides decreases below room temperature with increasing nitrogen content, a broad central doublet (QS = 0.57 mm/s and IS = 0.3 mm/s) might be assigned to ~-Fe~N with 2 < x < 3, while a second Fe phase (QS = 0.22 mm/s and IS = 0.2 mm/s) has a small contribution which seems to be rather constant during deposition. All C E M S spectra recorded from the same systems are similar to that plotted in Fig. lf, showing a higher relative contribution form the ferromagnetic Fe nitride phase than the average derived from the transmission spectra and, as a striking feature, the virtual absence of any paramagnetic contribution in the upper F e T a N layer, up to the relevant depth of about 3000 A. The phase analysis results for both the transmission and C E M S spectra are represented in Fig. 2, in terms of
the nitrogen partial pressure (p.~.:t during the FeTaN layer deposition. While the relative content in metallic Fe and ferromagnetic Fe nitride derived from the C E M S spectra is rather independent of the deposition conditions (up to 3000 A depth), the transmission spectra show instead an increase in the relative ~-FexN (2 < x < 3) fraction, that is proportional to the square of pN_,. This implies that the paramagnetic Fe nitride is formed at the interface region between the 3200A thick magnetic layers F e T a N and the T a N spacer layers. The positive magnetostriction of the system was found to be of the order of 5 x 10 ~ for P,v_, = 0.12 m T o r r [2] and has to be minimized for the improvement of magnetic sensor response. Since magnetostriction increases with increasing N2 content and formation of nitride phases [5,6], it is reasonable to assume that the interfacial a-FexN (2 < x < 3) phase, revealed by the M6ssbauer spectra, may account for this relatively high magnetostriction, that consequently might be lowered by controlling the deposition conditions for the soft F e T a N layers.
References [1] G. Qiu, J.A. Barnard, J. Appl. Phys. 75 (1994) 6934. [2] S.X Li, P.P. Freitas, M.S. Rogalski, M. Azevedo, J.B. Sousa, Z.N. Dai, J.C. Soares, N. Matsakawa, H. Sakakima, J. Appl. Phys. 81 (1997) 4501. [3] S.X. Li, P.P. Freitas, S. Cardoso, J.C. Soares, B. Almeida, J.B. Sousa, unpublished. [4] D.H. Mosca, P.H. Dionisio, W.H. Schreiner, I.J.R. Baumvol, C. Achete, J. Appl. Phys. 67 (1990) 7514. [5] J.C. Cates, C. Alexander Jr., E. Haftek, J.A. Barnard, IEEE Trans. Magn. Mag. 29 (1993) 3105. [6] Y. Takeshima, N. Ishiwata, T. Korenari, H. UraL J. Appl. Phys. 73 (1993) 6576.
Journal of Magnetism and Magnetic Materials 196-197 (1999) 75-76
M6ssbauer investigation of FeTaN/TaN multilayer systems M.S. Rogalski a'b'*, M.M. Amado a, J.B. Sousa a, P.P.
Freitas c
alFIMUP and Faculdade de Ciencias, Universidade do Porto, Rua Campo Alegre 687, 4150 Porto, Portugal bUCEH, Universidade do Algarve, Gambelas, 8000 Faro, Portugal ClNESC and Instituto Superior Tbcnico, Rua Alves Redol 9, 1000 Lisbon, Portugal
Abstract
The microstructure of the [FeTaN(3200,~)/TaN(50 A)], multilayer system, prepared by DC reactive magnetron sputtering, is studied by transmission and conversion electron M6ssbauer spectroscopy, that indicate a predominant a-Fe texture and the presence of iron nitrides. Comparative phase analysis reveals formation of the paramagnetic nitride a-FexN(2 < x < 3) in the interfacial region between the FeTaN and TaN spacer layers. This phase, whose relative fraction increases in proportion to the square of Nz partial pressure during deposition, may account for the relatively large magnetostriction of the multilayer system. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Ferromagnetic multilayers; Conversion electron-MiSssbauer; Thin films-M6ssbauer spectroscopy; Magnet-
ronsputtering
FeTaN films exhibit a combination of high saturation magnetization and good soft magnetic-behaviour, which is suitable for pole piece materials of high density recording heads. Improved magnetic properties over single layer films have been found for multilayers, where FeTaN films are separated by high resistivity spacer layers [1]. Recently, a new multilayer system [FeTaN/TaN],, deposited by DC reactive magnetron sputtering, was reported as a good candidate for high magnetization pole piece fabrication, provided magnetostriction is lowered [2]. In the present paper, microstructure of [FeTaN/ TAN], multilayers was studied by transmission and conversion electron (CEMS) M6ssbauer spectroscopy and was found to be rather sensitive to the nitrogen partial pressure during deposition of soft FeTaN layers. The [FeTaN(3200 ~)/TaN(50 ~,)], multilayers were deposited, up to a total thickness of 1.6 ~tm, on water cooled mica substrates, using a Nordiko 2000 sputtering system. For FeTaN film deposition, 3 in Fe-10 wt% Ta alloy targets were used, in 3 mTorr Ar + Nz atmosphere, * Corresponding address. UCEH, Universidade do Algarve, Gambelas, 8000 Faro, Portugal, Fax: + 351-89-818560; e-mail: [email protected].
at variable nitrogen partial pressure PN2 from 0 to 0.12mTorr. An aligning field of 20 Oe was applied. High-resistivity TaN films were deposited from 3 in Ta targets in 5 mTorr Ar + N2 atmosphere, at constant PN2 = 0.19 mTorr. Transmission STFe M6ssbauer spectra were obtained at room temperature on a Wissel constant acceleration spectrometer which utilized a 57Co/Rh source. The CEMS measurements were performed at room temperature by using an He-4% CH 4 gas-flow electron counter (Rikon-5 Wissel). The resulting spectra were calibrated with a-Fe foil at room temperature and fitted using an integrated least-squares computer program. The information provided by the transmission M6ssbauer is integrated over the five FeTaN layers, including the interfacial regions between the magnetic and spacer layers, while that derived from the CEMS spectra is limited to a maximum depth of 3000 ~,, and hence is relevant for the upper FeTaN layer structure only. All transmission M6ssbauer spectra, given in Fig. la-e, show a dominant contribution from the magnetic sublattice with H F of 334 kOe, typical for a-Fe, with spins oriented in the plane of the FeTaN layers, as indicated by the 3 : 4 : 1 : 1 : 4 : 3 relative intensity ratio.
0304-8853/99/$- see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 85 3(9 8 ) 0 0 6 6 7 - 2
76
M S. Rogalski et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 75- 76
•
100
°
(a)
(b)
1.02 .Co~ 8 0 ! "
1.00 1.00 >,0.99
oo
1.00 '.;±-
~
60- • O 40 L V
• •
• •
,~Fe FeN (x>3) ,-Fe N (2
•
• •
• O
,~Fe FeN (x>3)
O
O O
• •I
I
-;':-. -=~
~> 0.99
20~O
(d)
oi_
Oo .
0 "~O 0'04
lO0
~
O O
-
0 "08 I O: ~ ~ 0 I00 0"64 0'68
0'12
PN [mTorr]
0.99 ,
1.00 ~ .:-...
b
"
.
"
0,99
,
L
.
.
~
I
,
,
~
Fig. 2. Fe phase concentration vs. nitrogen partial pressure PN, during the FeTaN layer deposition, calculated fi-om (a) transmission M6ssbauer spectra and (b) conversion electron M6ssbauer spectra.
1.00
0.99 -8
-6
-4
-2
0
2
4
6
Velocity [rnm/s]
Fig. 1. Transmission M6ssbauer spectra recorded from FeTaN/TaN multilayer system deposited at variable nitrogen partial pressure: (a) Pu~ = 0; (b) PN~ =0.05mTorr; (c) p.,~, = 0.07 mTorr; (d) PN~ = 0.10 mTorr; (e) p~ = 0.12 mTorr and the corresponding CEMS spectrum from the system deposited at p,~.: = 0.12 mTorr (f). Larger linewidths than those expected for metallic Fe are consistent with a spreading in magnitude of the H F values, which is consistent with the fine grain size calculated from the X-ray diffraction patterns [3]. A second magnetic phase, with H F = 295 kOe can be identified as iron nitride FexN, which is ferromagnetic at room temperature for x > 3 [4] and contains Fe magnetic moments also oriented in the plane of the film. Since the Curie temperature of Fe nitrides decreases below room temperature with increasing nitrogen content, a broad central doublet (QS = 0.57 mm/s and IS = 0.3 mm/s) might be assigned to ~-Fe~N with 2 < x < 3, while a second Fe phase (QS = 0.22 mm/s and IS = 0.2 mm/s) has a small contribution which seems to be rather constant during deposition. All C E M S spectra recorded from the same systems are similar to that plotted in Fig. lf, showing a higher relative contribution form the ferromagnetic Fe nitride phase than the average derived from the transmission spectra and, as a striking feature, the virtual absence of any paramagnetic contribution in the upper F e T a N layer, up to the relevant depth of about 3000 A. The phase analysis results for both the transmission and C E M S spectra are represented in Fig. 2, in terms of
the nitrogen partial pressure (p.~.:t during the FeTaN layer deposition. While the relative content in metallic Fe and ferromagnetic Fe nitride derived from the C E M S spectra is rather independent of the deposition conditions (up to 3000 A depth), the transmission spectra show instead an increase in the relative ~-FexN (2 < x < 3) fraction, that is proportional to the square of pN_,. This implies that the paramagnetic Fe nitride is formed at the interface region between the 3200A thick magnetic layers F e T a N and the T a N spacer layers. The positive magnetostriction of the system was found to be of the order of 5 x 10 ~ for P,v_, = 0.12 m T o r r [2] and has to be minimized for the improvement of magnetic sensor response. Since magnetostriction increases with increasing N2 content and formation of nitride phases [5,6], it is reasonable to assume that the interfacial a-FexN (2 < x < 3) phase, revealed by the M6ssbauer spectra, may account for this relatively high magnetostriction, that consequently might be lowered by controlling the deposition conditions for the soft F e T a N layers.
References [1] G. Qiu, J.A. Barnard, J. Appl. Phys. 75 (1994) 6934. [2] S.X Li, P.P. Freitas, M.S. Rogalski, M. Azevedo, J.B. Sousa, Z.N. Dai, J.C. Soares, N. Matsakawa, H. Sakakima, J. Appl. Phys. 81 (1997) 4501. [3] S.X. Li, P.P. Freitas, S. Cardoso, J.C. Soares, B. Almeida, J.B. Sousa, unpublished. [4] D.H. Mosca, P.H. Dionisio, W.H. Schreiner, I.J.R. Baumvol, C. Achete, J. Appl. Phys. 67 (1990) 7514. [5] J.C. Cates, C. Alexander Jr., E. Haftek, J.A. Barnard, IEEE Trans. Magn. Mag. 29 (1993) 3105. [6] Y. Takeshima, N. Ishiwata, T. Korenari, H. UraL J. Appl. Phys. 73 (1993) 6576.